CSE 131 B Compiler Construction II Discussion 4

CSE 131 B – Compiler Construction II Discussion 4: Project 1 1/31/2007

Overview w Arrays w Pointers w Records w Topics/Questions you may have

Array Declaration w Syntax: VAR List 1 : ARRAY Index OF Type; VAR List 2 : ARRAY Index OF Type; w What to check in the declaration: n n Index must be an INTEGER Index > 0 (known at compile time Const. STO)

Array Usage w Syntax: my. Array[index]; (my. Array[index])[index]; (* notice how this still works *) w What to check: n n my. Array must be a variable (Var. STO) and of Array. Type index must be equivalent to INTEGER

Multidimensional Arrays Given: VAR x : ARRAY 4 OF ARRAY 3 OF INTEGER; a 0 b 1 a 1 b 2 b 0 b 1 a 3 b 2 b 0 b 1 b 2 Because of this, the following is valid: TYPE my. Arr = ARRAY 3 OF INTEGER; Remember: VAR x : ARRAY 4 OF my. Arr; The result of an array element is ADDRESSIBLE VAR y : my. Arr; y : = x[2];

Implementation? w How can one encapsulate the information from the array declaration for later use? w Remember the Type Hierarchy: n n One possibility is to store information such as element. Type, dim. Size. In order to do this, you have to add these fields into the Array. Type definition and provide ways to set and read the information from them.

Further Analysis of Arrays VAR my. Array : ARRAY 20 OF INTEGER; VAR x, y : INTEGER; BEGIN my. Array[x] : = my. Array[4]; my. Array[x+2 -y] : = 15; my. Array : = 10; (* error, not assignment compatible *) END.

Pointer Declaration w Syntax: VAR x : POINTER TO Type; TYPE ptr = POINTER TO Type; VAR y : ptr; w What to check in the declaration: n Type must be a RECORD

Pointer Usage w Syntax: ptr. (* pointer dereference, like “ *ptr ” in C *) -- T_DOT NEW ptr; w What to check: n n For a dereference (check 15), only things that are variables of pointer to records can be dereferenced. For New (check 18), you must check to see if the argument is also of Pointer. Type.

Further Analysis of Pointers TYPE ptr = POINTER TO RECORD a : REAL; END; VAR x, y : ptr; VAR z : REAL; BEGIN NEW x; (* like malloc, no actual allocation in Proj 1 *) x. a : = 7; (* assign value of 7 into where x’s a field *) y : = x; (* assign y to point where x points *) z : = y. a; (* get y’s a field and put it into z *) END. Remember, no actual assignment of data here – that’s the next proj

Record Declaration w Syntax (think of C structs): VAR r : POINTER TO RECORD a, b : INTEGER; b : REAL; END; w What to check in the declaration: n n Check for duplicate fields (field r. b in this case) If a field is duplicated multiple times, an error is reported for each duplicate instance.

Record Usage w Syntax: my. Rec. my. Field w What to check: n n n my. Rec must be a Var. STO of some pointer to a record my. Rec must contain the field (my. Field, in this case) After checking, your result may be a Var. STO of the type of my. Field, since you want something ADDRESSIBLE

Further Analysis of Records TYPE rec = RECORD a, b : REAL; END; VAR my. Rec : POINTER TO rec; BEGIN my. Rec : = my. Rec; (* fine – assignable pointers *) my. Rec. b : = 3. 6; (* assign 3. 6 into b field *) my. Rec. a : = my. Rec. b; (* assign b field into a field *) END.

Recursive Records w Recursive types are easily possible, since there will always be at least one POINTER type in a cycle. w When you encounter a Type declaration, you want that Type to be in scope immediately, even if you are not finished with the Type declaration (ie, it isn’t fully complete, but can still be referenced from the Symbol Table).

Recursive Record Examples TYPE linked. List = POINTER TO RECORD a : INTEGER; next : linked. List; END; TYPE rec = RECORD a : POINTER to rec; END;

Recursive Records w Think more about this – it is an important concept (not just for this class). w Some ideas: n n Put a partially complete/empty Type. Alias. STO onto the Sym. Tab before the actual type is know. Have a stack of items that need to be re-checked at the end of the Type. Decl, and assume everything is fine until you reach the end of the declaration, then go through and re-check based on the stack.

Refresher on Type Equivalence w Remember the following rules: n n All basic types use structural equivalence Arrays and Records use strict name equivalence Pointers use whatever the type they point to Type aliases use loose name equivalence

Illustrative Example TYPE int = INTEGER; TYPE month = INTEGER; VAR i : int; VAR m : month; VAR r : REAL; TYPE rec = POINTER TO RECORD a : REAL; END; VAR r 1 : POINTER TO RECORD a : REAL; END; VAR r 2 : rec; FUNCTION f(VAR a : rec); (*. . . *) END f; FUNCTION ff(a : rec); (* … *) END ff; VAR a 1 : ARRAY 5 OF REAL; VAR a 2 : ARRAY 5 OF REAL; FUNCTION g(a : ARRAY 5 OF REAL); (*. . . *) END g; VAR p 4 : POINTER TO RECORD a : REAL; END; BEGIN i : = m; (* okay, loose name equivalent *) i : = r; (* error, not assignable *) r : = i; (* okay, assignable *) f(r 2); (* okay, same type *) f(r 1); (* error, not strict name equivalent *) ff(r 2); (* okay, same type *) ff(r 1); (* error, not strict name equivalent *) g(a 1); (* error, not strict name equivalent *) gg(a 1); (* error, not strict name equivalent *) a 1 : = a 1; (* okay, strict name equivalent *) a 1 : = a 2; (* error, not strict name equivalent *) r 1 : = r 1; (* okay, strict name equivalent *) r 2 : = p 4; (* error, not strict name equivalent *) END. For more info, see Appendix A of the Oberon spec